The present invention relates to a method and apparatus for detecting that two moveable members are correctly positioned relatively to one another.
The invention has a particular application to an ion implantation apparatus in which a single wafer is transferred in a vacuum chamber from a loading mechanism such as loadlock to an electrostatic chuck (hereinafter referred to as an e-chuck) on a gripper arm. The gripper arm also retrieves the wafer from the e-chuck and returns it to the loadlock.
Typically, a robot which drives the gripper arm will have between two and four axes of movement. For example, EP 604066 discloses a gripper arm which is axially movable along a vertical axis and rotatable about the same axis.
The e-chuck will typically have several axes of movement, for example, as disclosed in WO 99/13488. In this case the arm which supports the e-chuck extends out of the vacuum chamber and is supported by a linear motion mechanism for reciprocably moving the e-chuck vertically so that the entire surface of a wafer on the e-chuck is scanned by a horizontally scanning ion beam. The linear motion mechanism itself is mounted so as to be rotatable about a horizontal tilt axis which allows the angle between the wafer and the ion beam to be varied. The e-chuck is further provided with a mechanism for rotating the wafer about an axis passing through a centre of the wafer and perpendicular to the plane of the wafer.
Although the mechanisms driving the gripper arm and the e-chuck are provided with encoders which allow them to be driven to a very precise position, the fact that there are so many axes of freedom means that, over time, the mechanisms may drift, particularly as components of the mechanisms start to wear. This results in an imprecise handoff between the gripper arm and e-chuck such that the wafers have to be slid over the surface of the e-chuck causing particulate contaminants on the underside of the wafer. Also, as the encoders cannot provide an absolute position reading they have to be calibrated before use. Such calibration is normally done manually, which can be particularly time consuming and requires a skilled operator.
According to the present invention there is provided an apparatus for detecting that two moveable members are correctly positioned relatively to one another in a predetermined position, the apparatus comprising a light source, a light sensor onto which light from the light source is projected, and a respective flag on each member which produces a characteristic shadow on the light detector when in the predetermined position, the characteristic shadows of the two members being different from one another.
This apparatus will accurately verify the relative position of the two members, as both members are located with respect to the same light source. As optical sensors are being used, they provide an indication of the absolute position of the two members and are independent of the mechanism driving the two members.
Preferably, the flag on each member is a through hole, with the through hole on one member being larger than the through hole on the other, and wherein the centres of the through holes are aligned in the direction in which the light from the light source travels to the sensor when the members are in the predetermined position. The sensor is preferably a two dimensional position sensitive diode. The light source can be any collimated light source, but is preferably a class 1 modulated infra red laser as the sensor can distinguish between this and ambient light and it also satisfies the necessary safety requirements.
The present invention is particularly applicable to an ion implantation apparatus, wherein the first member is a gripper arm which transfers a wafer within a vacuum chamber from a loading position to a processing position and back, and the second member is an e-chuck on which the wafer is retained while it is scanned by the ion beam. In this case, the flags should be provided on the e-chuck and the gripper arm so as to have a fixed relationship with the wafer position to allow the position of the wafer to be determined easily from the sensor readings. Generally speaking, the e-chuck is brought to the handoff position before the gripper arm. Therefore, when the flags on the two members are provided by through holes, the diameter of the through hole associated with the e-chuck is greater than the diameter of the through hole associated with the gripper arm. When the light source and light sensor are mounted outside the vacuum chamber behind appropriately positioned windows they can be readily removed for maintenance or replacement without requiring access to the vacuum chamber.
When a gripper arm is used which has a pair of jaws which are moveable between open and closed configurations, the gripper arm may be provided with a third flag which produces a third characteristic signal on the light sensor when the gripper arm is in the predetermined position and its jaws are open, allowing the apparatus to determine the configuration of the gripper jaws.
According to a second aspect of the present invention, there is provided a method of detecting that two moveable members are correctly positioned relatively to one another at a predetermined position, the method comprising emitting a beam of light from a light source towards a light sensor, moving each member, so that a flag on each member produces a characteristic shadow on the light detector when in the predetermined position, and detecting that the two members are in the correct alignment when both characteristic shadows are simultaneously recognised on the light sensor.
The present invention also extends to a method of calibrating an apparatus having a pair of moveable arms which are driveable to a handoff position to pass a component from one arm to the other, a position sensitive light sensor, and a light source emitting light towards the light sensor, wherein each arm has a flag which produces its own characteristic signal at the light sensor when interposed between the light source and the light sensor, the method comprising the steps of scanning the first arm across the sensor, recording the readings produced by the first flag across the sensor, scanning the second arm across the sensor, recording the readings produced by the second flag across the sensor, and calculating from the two sets of readings the position where the two flags coincide which represents the handoff position.
This method allows for the automatic calibration of the handoff position which can be done in software. Further, precise positioning of the light source and sensor is unnecessary as any inaccuracies in their positioning are calibrated out using this method.
Preferably, the two moving arms are robot arms driven by one or more servo motors each having an encoder, and the method further comprises noting the encoder readings at the handoff position, and subsequently driving the robot arms to the positions derived from encoder readings at the handoff position, and detecting the signal produced by the sensor at this position. This method allows the position of the arms to be monitored, such that any drift in the position of these arms can be detected.
The present invention further encompasses an apparatus for calibrating and controlling the handoff between a gripper arm and an electrostatic chuck in an ion implantation apparatus, the gripper arm and electrostatic chuck each being driven by a one or more servo motors, the apparatus for calibrating and controlling comprising a computer, an encoder on each servo motor for producing a signal relating to the position of the servo motor and providing this information to the computer, a connection between the computer and a drive mechanism of each servo motor for controlling the motion of each servo motor, and a sensor for receiving light from a light source; the computer having a processor arranged to receive and store values indicative of signals from the encoders and sensor recorded when the gripper arm and electrostatic chuck are at a predetermined handoff position at which the gripper arm and electrostatic chuck intercept a proportion of the light directed to the sensor and produce a characteristic signal at the sensor indicative of the fact that the gripper arm and electrostatic chuck are at the handoff position, to subsequently output a signal to the drive mechanism of each servo motor to drive each servo motor to a position at which the reading from each encoder matches the stored value, and to record the signal from the sensor in this position and compare this value with the stored value.
Preferably the processor is further arranged to determine the drift of the gripper and electrostatic chuck by comparing the stored value of the sensor with each measured value. Preferably the processor is further arranged to alert an operator if the amount of drift detected exceeds a predetermined threshold.